1. Field of the Invention
The present invention relates in general to electrical devices for measuring and indicating the phase angle between two signals and, more particularly, to devices for measuring and indicating the power factor derived from a detected current and voltage.
2. The Prior Art
The total electrical energy that is necessary to operate the appliances in a home or the machinery in a factory typically consists of two components. One of the components is termed "average power." The average power is the electrical energy actually expended by an electrical system in driving the appliances or machinery in use. The other component is called "reactive power" or "reactive volt-amperes," commonly abbreviated as "vars." Reactive power is electrical energy that is stored in the capacitive or inductive elements of a circuit. Reactive power represents an inefficiency in terms of the total electrical energy available for productive use by an electrical system.
The relationship between the average power and reactive power may be given by the complex equation EQU P=VI cos .theta.-jVI sin .theta.
where P represents the total complex power and where V represents the AC voltage, I represents the AC current, and .theta. represents the phase angle between the voltage and current.
In the above equation VI cos .theta. represents the average power, whereas VI sin .theta. represents the reactive power. The quantity cos .theta. is referred to as the power factor.
The efficiency of an electrical system may readily be determined by examining the angle between the voltage and current. When the power factor (i.e. cos .theta.) is 1 (i.e. when .theta.=0 degrees and the current and voltage are in phase) an electrical system will be at peak efficiency. As .theta. approaches 90 degrees the system's inefficiency due to vars will progressively increase. Maximum inefficiency loss due to vars occurs where cos .theta.=0 (i.e. where the current and voltage are 90 degrees out of phase). In an inductive circuit, the angle .theta. will be negative and the current will lag the voltage. In a capacitive circuit the phase angle will be positive and the current will lead the voltage.
By knowing the magnitude of the angle .theta. and whether .theta. is leading or lagging, the efficiency of an electrical system may be corrected by adding capacitance to an inductive circuit or by adding inductance to a capacitive circuit. In this way the efficiency of an electrical system may be significantly increased.
With the growing emphasis on energy conservation, utility companies are becoming increasingly aware of the power factor of the electrical systems of their customers. It is not uncommon for utility companies to charge higher rates to customers, particularly industrial or commercial customers, that have electrical systems having a low power factor. Accordingly there is an increasing need to be able to easily and quickly measure the power factor of an electrical system in order to determine and improve its efficiency.
Various types of prior art devices have been developed which have been used for purposes of measuring the power factor of an electrical system. One such prior art device is electromechanical in its nature. This type of device uses one fixed coil and one movable coil and determines the phase angle and hence the power factor from the amount of torque exerted on the movable coil. Although reliable, this type of device is bulky and relatively expensive to manufacture.
With the advent of computer technology, more sophisticated kinds of power factor metering devices have been devised. See, for example, U.S. Pat. Nos. 2,970,263 and 3,084,863.
Various types of phase metering devices have also been developed for purposes of measuring the phase difference between communication signals such as commonly used in radio or television. See, for example, U.S. Pat. Nos. 3,358,231, 3,631,340, 3,725,781, 3,787,765, 3,805,153, 3,984,701 and 4,025,848.
Although each of the above-indicated types of prior art phase metering devices may work well for their intended applications, to date there has not been developed an efficient and portable power factor metering device that is compact, economical and that conveniently provides all the necessary power factor information derived from an electrical system.
Thus, what is needed is a compact, portable power factor metering device that is economical and reliable in its operation. The power factor metering device should be operable at various levels of input voltage and should provide an instantaneous power factor readout so that the power factor of a system can be spot checked at any time. The power factor metering device should also provide a continuous recording device so that the nature of electrical loads can be analyzed under various conditions and over a period of time. The device should also be capable of indicating whether a particular system is leading or lagging so that the proper correctional measures can be taken to improve the power factor. Such an invention is disclosed and claimed herein.